The present invention relates to a scanning optical apparatus for performing recording and reading of a high-precision fine image and, more particularly, to a scanning optical apparatus for detecting a focusing position of a beam spot in a direction perpendicular to a scanning surface formed by a scanning beam as a function of time.
In a conventional laser scanning apparatus, it is known to control movement of a light source or lens by an actuator (e.g., Japanese Laid-Open Patent Application No. 59-116603) to correct an error of a focusing position of a laser beam.
In a conventional optical disk memory system, it is also known to control a pickup lens so as to automatically focus a beam on an optical disk surface in an in-focus state (auto focus) or to automatically radiate a beam on a target track (auto tracking).
In the former prior art, in order to correct an error of a focusing position of a laser beam, when the light source or lens is controlled to be moved in an optical axis direction, the laser beam is deviated in a direction perpendicular to the scanning surface, and a positional error of the laser spot in this direction may occur.
When a rotary polygonal mirror or vibration mirror is used as a light deflector, a laser beam may be deviated in a direction perpendicular to the scanning surface due to a machining error of its deflection/reflecting surface or instability of a rotating shaft of the deflector.
In the latter prior art, i.e., in an optical disk memory, light information is recorded upon rotation of a recording medium, i.e., an optical disk. A light deflector is not used unlike in the above scanning optical system. In the optical disk memory, a positional relationship between a light source and a focusing point on the optical disk is basically kept unchanged. Auto focus (AF) and auto tracking are performed to cope with only variations (eccentricity and inclination) of the optical disk surface. Since these variations, however, are very small, it is not easy to apply the AF and AT techniques to the former prior art, i.e., the laser scanning optical system and to correct the positional error due to differences in arrangements.
In other words, the positional relationship between the light source and the focusing point on the surface of the recording medium such as a photosensitive body is changed over time with a cyclic movement of the light deflector, unlike in the optical disk memory system. Control must be concentrated on the positional error of the focusing point.
In a scanning optical apparatus for scanning and exposing a recording body surface with a light beam, in order to obtain a high-quality image, the focusing point of the light beam is preferably always located at a predetermined position from the surface of the recording body. However, the focusing position of the light beam does not always maintain a predetermined positional relationship due to aberration, thermal expansion, and the like.
For this reason, there is provided a system for maintaining the position of a focusing point at a predetermined position by driving the light source or a part of the optical system (e.g., Japanese Laid-Open Patent Application No. 63-78167).
Of a copying machine, a recording apparatus, and the like using laser beams, an apparatus using a rotary polygonal mirror may have irregular surface inclination of the polygonal mirror to cause feed pitch errors in scanning lines, and a high-quality image may not be obtained. In order to solve this problem, a system for correcting irregular inclination by a surface inclination correcting optical means is known (Japanese Patent Publication No. 52-28666).
The optical surface inclination correcting system has disadvantages in that scanning line feed pitch errors caused by irregular feeding of a recording body cannot be corrected, and that an optical system is complicated and expensive. For this reason, a non-optical surface inclination correcting system is also proposed (Japanese Laid-Open Patent Application No. 59-15217).
In a conventional scanning optical apparatus for scanning and exposing a recording body surface with a light beam, in order to maintain a scanning position of the light beam at a predetermined position on the recoding body surface, there is provided a method of driving a part of an optical system or a reflection mirror or the like arranged in part of an optical system (e.g., Japanese Laid-Open Patent Application Nos. 61-190311, 61-278814 and 59-15217).
At this time, information used to control part of the optical system is obtained from a means for detecting a feed error of a recording body or a means for storing surface inclination information for a polygonal mirror serving as a deflector (e.g., Japanese Laid-Open Patent Application Nos. 59-15217 and 61-190311).
A method of maintaining a focusing point of a light beam at a predetermined position from the surface of a recording body during exposure and scanning is a method of driving the light source or a part of the optical system (e.g., Japanese Laid-Open Patent Application No. 63-78167). At this time, information for controlling the part of the light source or optical system is information obtained by a photodetector arranged in part of the optical system or information stored in the storage means (e.g., Japanese Laid-Open Patent Application Nos. 60-100113 and 61-25367).
The following problems, however, are posed by the above conventional methods. For example, in control for maintaining the focusing position of the light beam or the scanning position at the predetermined position in accordance with information from the storage means, if focusing or scanning positional errors are caused by information except for that stored in the storage means, correction control cannot be performed.
When correction control is to be performed on the basis of information from the photodetector arranged in part of the optical system or the means for detecting irregular feeding, it is very difficult to perform correction control of focusing and scanning positional errors for all scanning lines due to structural limitations. At the same time, an operation from information detection to error correction must be performed almost in real time. This operation is very difficult when processing time and the like of an electronic circuit are also taken into consideration.
Japanese Laid-Open Patent Application No. 62-225067 proposes an optical scanning apparatus using two photodetectors constituting a means for detecting a change in scanning line position caused by a surface inclination error of a rotary polygonal mirror. These photodetectors are spaced apart from each other by a predetermined distance in a direction perpendicular to a scanning surface. Japanese Laid-Open Patent Application No. 55-74519 proposes an optical scanning apparatus for detecting a shift amount of a scanning light beam as follows. At the time of measurement of a shift of a scanning light beam caused by a surface inclination error of a rotary polygonal mirror, a solid image pickup element array arranged in a shift measurement direction of the scanning light beam is arranged such that a plurality of solid image pickup elements are arranged at a very small pitch. These elements are irradiated with the scanning light beam, and the shift amount of the scanning light beam is detected by a distribution of outputs from the solid image pickup elements.
In the above prior art, since one detector is arranged within the scanning range of the scanning light beam, i.e., since one detector is used in the scanning direction of the light beam, it is impossible to perform correction control of scanning positional errors along the entire scanning line. In Japanese Laid-Open Patent Application No. 62-225067, a difference between light amounts received by the two photodetectors is used as surface inclination information of a rotary polygonal mirror. In Japanese Laid-Open Patent Application No. 55-74519, a correspondence between a scanning light beam and the order of an ON element of a solid image pickup element array is detected, and the detection information is used as surface inclination information of the rotary polygonal mirror. For this reason, when the scanning direction of the scanning lines is not inclined from a scanning direction (ideal scanning direction) at the time of recording or reading and is vertically shifted to be parallel thereto, correction of the scanning line position can be performed. However, when the scanning direction of the scanning line is inclined from the scanning direction (ideal scanning direction) of recording or reading and is vertically shifted not to be parallel thereto, correction of the scanning line is impossible.
It is, therefore, an object of the present invention to provide a scanning optical apparatus which can appropriately control a positional error of a beam spot on a scanning surface so as to solve the conventional problems described above.
In order to achieve the above object of the present invention, there is provided a scanning optical apparatus comprising means for detecting a scanning positional error of a beam spot on a scanning surface such as an original or a photosensitive body in a direction perpendicular to the scanning surface of the light beam, the original being recorded with character image information, and means for controlling movement of the beam spot in the direction perpendicular to the scanning direction, wherein the control means controls movement of the spot position so as to always scan the beam spot in a predetermined mode in accordance with the magnitude and a change as a function of time of a signal from the detecting means.
The means for detecting the scanning position of the beam spot on the scanning surface further has a function of detecting a focal point or focusing position of a beam within the scanning surface of the beam. In this case, a positional error of the beam spot in the direction perpendicular to the scanning surface and a positional error of a beam waist in a direction perpendicular to the direction of beam scanning within the scanning surface can be independently corrected.